Charles Moore wrote a great article about the unibody 13″ MacBook compared to the much-loved 12″ PowerBook. A friend of Charles argued that until the dimensions were nearly identical it could never be considered a replacement. Charles feels there’s a little more to it than that.
I think they’re both right (yes, life is good sitting on top of this fence).
I don’t disagree with Charles’ friend that width is a big factor, and here the new MacBook is much bigger than the 12.” However, I would suggest that depth is the more critical (for use on a table, airline tray table, etc.) and here the new model is only slightly bigger. Further, weight is a big factor and the two are pretty much identical.
So you need to consider just what you’re getting for those extra couple inches of width…